Extrusion apparatus for use in the production of thermoplastic resin foams

ABSTRACT

An extrusion apparatus including an extruder comprising an extruder barrel, an extruder screw and an injection port for blowing agent and a cooling device connected to the forward end of said extruder, said extruder screw consisting of a compression zone wherein a volume in unit pitch of the screw decreases progressively in the extrusion direction and a metering zone wherein a volume in unit pitch of the screw is 1.6 - 2.0 times a volume in the final unit pitch of the screw in the compression zone and is constant in the extrusion direction, said injection port being provided above the metering zone, and said cooling device consisting of a coolant vessel disposed coaxially with said extruder, a plurality of pipes in said coolant vessel for dividing the flow of resin spaced apart from each other, an adapter for conducting the flow of resin from said extruder to said pipes, and an adapter for gathering the divided flows of resin extruded from said pipes.

nited States Patent [191 awai et a1.

[ EXTRUSION APPARATUS FOR USE IN THE PRODUCTION OF THERMOPLASTIC RESINFOAMS [75] Inventors: Isami Kawai, Ageo; Atsuro Nishikawa, Osaka; AkiraIwata, Osaka; Kohei Sugiyama, Osaka, all of Japan [73] Assignee: SekisuiKagaku Kogyo Kabushiki Kaisha, Osaka, Japan [22] Filed: June 22, 1970[21] Appl. No.: 47,980

[52] U.S. Cl 425/4 C, 259/9, 264/51,

[51] Int. Cl B29d 27/00 [58] Field of Search 18/12 SE, 12 SJ, 12 SN,

18/12 SS, 12 TT; 425/4, 4 C, 817,817 C, 208; 264/51, 53, 328, 329;259/5, 9, 104, 191

[56] References Cited UNITED STATES PATENTS 3,160,688 12/1964 Aykanianet al 18/12 SN X 3,391,051 7/1968 Ehrenfreund et al l8/12 SN X 3,431,5993/1969 Fogelberg 18/12 SN -11 l l l5 I6 3,827,841 Aug. 6, 1974 57]ABSTRACT An extrusion apparatus including an extruder comprising anextruder barrel, an extruder screw and an injection port for blowingagent and a cooling device connected to the forward end of saidextruder, said extruder screw consisting of a compression zone wherein avolume in unit pitch of the screw decreases progressively in theextrusion direction and a metering zone wherein a volume in unit pitchof the screw is 1.6 2.0 times a volume in the final unit pitch of thescrew in the compression zone and is constant in the extrusiondirection, said injection port being provided above the metering zone,and said cooling device consisting of a coolant vessel disposedcoaxially with said extruder, a plurality of pipes in said coolantvessel for dividing the flow of resin spaced apart from each other, anadapter for conducting the flow of resin from said extruder to saidpipes, and an adapter for gathering the divided flows of resin extrudedfrom said pipes.

6 Claims, 13 Drawing Figures PATENTED AUG 6 I974 SHEET 01 0F 10PATENTEDAUB 61974 sum 03 or 10 PATENIEU AUG 619M sum '05 or I b gm V r!v PATENTEU RUB 5 I974 sum as or 10 w E? mafia m 5 PATENTED AUG 6 I974SHEET 07 0F 10 llllllllllllllll'lllIl-I PATENTED AM; 61974 SHEET '09 or10 I M Emma k ll v i ra llllllllllllllllllllllllllll ll wm mv L Thisinvention relates to an extrusion apparatus for use in the production ofthermoplastic resin foams, and more particularly, to an extruder suitedfor extruding and foaming thermoplastic resins and a cooling deviceconnected to the extruder.

The well known method of producing a thermoplastic resin foam involveskneading and melting a thermoplastic resin within an extruder, adding avolatile organic liquid or gas as a blowing agent during kneading andmelting, cooling the molten resin containing the blowing agent to atemperature suitable for foaming, and extruding the cooled resin througha die to foam it. The most frequently used type of extruder for theproduction of foams of this kind is described in Japanese Pat.Publication No. 15712/68 filed in Japan by De Bell & Richardson Inc., onthe basis of US. Pat. Ser. No. 449,178 filed in the United States ofAmerica on April 19, 1965. This prior art extruder is illustrated inFIG. 8 of the accompanying drawings, and includes an extruder screwwithin an extruder barrel consisting of a compression zone A wherein avolume in unit pitch of the screw decreases progressively in theextruding direction, an intermediate metering zone B wherein a volume inunit pitch of the screw is equal to a volume in the last unit pitch ofthe screw in the compression zone, an enlarged zone C wherein a volumein unit pitch of the screw is larger than the volume in unit pitch ofthe screw in the intermediate metering zone, and a diminished meteringzone D wherein a volume in unit pitch of the screw is equal to a volumein the final unit pitch of the compression zone, a blowing agentinjection port being provided in the extruder barrel corresponding tothe enlarged zone C. When a resin foam is to be produced using such anextruder, the molten resin in the enlarged zone undergoes a greatresistance by the diminished metering zone adjacent to the enlargedzone, and stays at the enlarged zone C. The storage of the molten resinin the enlarged zone causes an irregular increase and fluctuation ofpressure with the degree of storage. Consequently, the amount of ablowing agent varies considerably with changes in the amount of themolten resin extruded, making it difficult to produce a thermoplasticresin foam having a fine cellular structure and a uniform density and auniform size.

Furthermore, when a thermoplastic resin is extruded and foamed by meansof the known extruder, it is not easy to cool the entire resin uniformlyto a temperature suitable for foaming, although when the output of theresin is relatively small, it is possible to lower the temperature ofthe resin by cooling the extruder barrel and screw. When the output ofthe resin becomes larger, mere cooling of the extruder barrel and screwis not enough to cool the resin to the desired foaming temperature, andit is almost impossible to maintain the entire molten resin uniformly ata temperature suitable for foaming; when the resin is extruded from adie, the gasified blowing agent escapes from the resin, resulting in asmaller ratio of expansion or a non-uniform foaming.

On the other hand, above-mentioned Japanese Pat. Publication No.15712/68 based on US. Pat. Ser. No. 449,178 discloses a cylinderco-axially connected to the forward end of an extruder of the known typedcscribed above. A plurality of pipes opened at both ends are disposedwithin the cylinder in a direction at right angles to the extrusiondirection, and adapted to cool the resin flowing in the cylinder to atemperature suitable for its foaming. When a resin foam is to beproduced by using such a cooling device, the resin flowing in thecylinder undergoes a large resistance by the pipes opened at both ends,and considerable amounts of resin tend to become stagnant around saidpipes. Because of this, it is difficult to extrude and foam the resinuniformly. Furthermore, because the resin undergoes resistance, too manypipes opened at both ends cannot be provided, and the area of contactbetween the resin and the pipes decreases; this naturally leads to arestricted cooling capacity. When the output the resin is increased, theresin cannot be throughly and uniformly cooled to the desired foamingtemperature, and foams of good quality cannot be obtained.

Accordingly, one object of the present invention is to provide anextruding apparatus for producing a thermoplastic resin foam with finecellular structure having a uniform density and a uniform size byinjecting a volatile liquid or gas as a blowing agent into molten resinat a relatively low pressure and extruding the molten resin containingblowing agent while preventing variations in injection pressure and inoutput of the resin to be extruded.

Another object of the invention is to provide a cooling device forproducing a thermoplastic resin foam with fine structure having auniform density and a uniform size by cooling molten resin containing avolatile liquid or gas as a blowing agent to a temperature suitable forits foaming with good efficiency and overall uniformity, followed byextrusion.

Other objects and advantages of the present invention will becomeapparent from the following description.

The above objects can be achieved by using an extrusion apparatusincluding an extruder wherein an extruder screw is fitted with anextruder barrel and a blowing agent injection port is provided in a partof said extruder barrel, said extruder screw consisting of a compressionzone in which a volume in unit pitch of the screw decreasesprogressively in the extrusion direction and a metering zone in which avolume in a unit pitch is 1.6 2.0 times a volume in the final unit pitchof the compression zone, said blowing agent injection port beingprovided at that part of the extruder barrel which corresponds to saidmetering zone. The above objects can also be achieved by using anextrusuion apparatus including an extruder having an extruder screw inan extruder barrel and a blowing agent injection port at a part of saidextruder barrel, and a cooling device connected to the forward end ofsaid extruder, said cooling device comprising a coolant vessel disposedco-axially with said extruder, a plurality of pipes for dividing theflow of resin spaced apart from each other and extending through thecoolant vessel in the extruding direction from one wall plate to theother wall plate thereof, an adapter for dividing the flow of resinextruded from the extruder and cause it to flow through said pipes, andan adapter for gathering the flows of the resin from the pipes.

In the present invention, it is necessary that a volume in unit pitch ofthe screw in the metering zone should be l.6 2.0 times a volume in thefinal unit pitch in the compression zone, and should be constant in theextrusion direction. This requirement has been found as a result of ourtheoretical and experimental studies of extrusion. If a volume in unitpitch of the screw in the metering zone exceeds two times a volume inthe final unit pitch in the compression zone, it is difficult tomaintain an output equilibrium of the resin between the compression zoneand the metering zone, resulting in a fluctuation in output of the resinextruded. Thus, the amount of blowing agent to be injected also varies,and the resin cannot be kneaded well with the blowing agent. Thisnaturally makes it impossible to form foams of good quality. On theother hand, if the volume in unit pitch of the screw in the meteringzone is below 1.6 times the volume in the final unit pitch in thecompression zone, it becomes difficult to maintain an output equilibriumof the resin between the compression zone and the metering zone,resulting in an undesirable rise in the pressure of the resin in themetering zone and the difficulty of injecting the blowing agent. Thus,the amount of the blowing agent to be injected varies, and foams of goodquality cannot be obtained. Generally speaking, if the volume in unitpitch of the screw in the metering zone is in the vicinity of 1.6 timesthe volume in the final unit pitch of the screw in the compression zone,the apparatus is suited for the production of resin foams with lowexpansion ratio, whereas if it is in the vicinity of 2.0 times, theapparatus is suitable for the production of resin foams with highexpansion ratio.

In the apparatus of the present invention, a volume in unit pitch of thescrew in the compression zone decreases progressively in the extrusiondirection in order to ensure a good kneading of the resin byheat-melting it while extruding it forwardly. The rear part of thecompression zone is used as a feed section for the resin. The volume inunit pitch of the screw can be varied either by changing the depth ofscrew channel while maintaining the screw pitch constant, or by changingboth the screw pitch and the depth of screw channel. The use of anextruder screw with constant pitch wherein a volume in unit pitch of thescrew is determined by changing the depth of the screw channel, gives asmooth flow of the resin and leads to optimum results. Furthermore, thiskind of extruding screw is easy to produce.

In general, the ratio of screw length (L) to screw diameter (D) is inthe range of 30 40. L/D in the compression zone is in the range of 1425, and L/D in the metering zone is in the range of l5. Compressionratio in the compression zone is in the range of 2 4.

In the apparatus of the invention, the injection port for a blowingagent is provided at a part of the exrruder barrel corresponding to themetering zone. The provision of the injection port an this placecontributes to the ease of injecting a blowing agent, and maintains aconstant rate of the blowing agent to be mixed with the resin.Furthermore, according to such a way of provision, the blowing agentdoes not flow back to the compression zone, and foams of good qualitycan be obtained. Especially good results will be obtained when theinjection port for a blowing agent is provided at that part of theextruder barrel which corresponds to the first one-half to one pitch ofthe screw.

In one preferred embodiment of the present invention, an intermediatemetering zone wherein a volume in unit pitch of the screw is equal to avolume in the final unit pitch of the screw in the compression zone isprovided between the compression zone and the metering zone. The resinfed from the compression zone is stabilized in the intermediate meteringzone and fluctuations in the injecting pressure of a blowing agent inthe metering zone is lessened, thereby giving foams of goods quality. Ingeneral, the L/D in the intermediate metering zone is 2 to 4.

In the apparatus of the present invention, a part of the metering zonemay be replaced with a dulmage type mixing torpedo wherein a volume inunit pitch is equal to a volume in unit pitch of the screw in themetering zone. A blowing agent injected is uniformly mixed with theresin by means of the dulmage type mixing torpedo to give foams of goodquality. The best results will be obtained if the first part of themetering zone is formed with this dulmage type mixing torpedo and ablowing agent is injected into this mixing torpedo. To the forward endof the extruder having the construction as described above, is connectedany extrusion die of an optional shape such as sheets, plates, tubes,pipes, rods, or filaments.

In the cooling device provided in the extrusion apparatus of theinvention, a box type vessel or a cylinder type vessel is used as thecoolant vessel. in the case of using the box-type vessel, it is possibleto fill the vessel with a suitable coolant such as water, oil andglycerine and control the temperature of the coolant to a constanttemperature, or to provide an inlet and an outlet for the coolant in thevessel and circulate the coolant maintained at constant temperature fromthe inlet to the outlet. When a vessel of cylinder type is used, theconstruction may be such that both ends of the cylinder are closed bymeans of wall plates and a coolant maintained at a constant temperatureis circulated from the inlet to the outlet for the coolant provided inthe cylinder. Alternatively, a coolant jetting pipe having a pluralityof openings for jetting out a coolant is provided along the central axisof the cylinder to extend from one wall plate to the other, and an inletfor the coolant which passes from both ends of the coolant jetting pipe,and through the inside of the wall plates and opens into the exterior isprovided in the wall plates, while an outlet for the coolant is providedin the cylinder to open into the exterior; thus, a coolant maintained atconstant temperature is caused to flow through the coolant jetting pipe,jetted out from a plurality of openings provided in the pipe, andcirculated to the output of the coolant.

The coolant vessel further contains a plurality of pipes for dividingthe flow of resin which extend in the extruding direction from one wallplate of the vessel to the other and disposed in spaced-apartrelationship. Since these pipes are spaced apart from each other in theextruding direction, the resin flowing through the pipes does notundergo any resistance. The pipes for dividing the flow of the resin arecooled by the coolant present in the coolant vessel, and the resinflowing in the pipes is maintained at a temperature suitable forfoaming.

The coolant vessel also includes an adapter for dividing a flow of resinextruded from the extruder into the pipes. The resin is smoothly dividedinto the pipes through the adapter. Another adapter for gathering theflows of resin extruded from the pipes for dividing the flow of resin isconnected to the coolant vessel. The flows of the resin are smoothlygathered through the adapter.

. 8 The cooling device described above is connected to the forward endof the extruder, and a die of any known type is secured to the forwardend of the cooling device to constitute the extrusion apparatus of thepresent in- I vention.

The above-mentioned cooling device is for cooling the flow of resindivided by the pipes to a temperature suitable for foaming, and thecooling of the resin is accomplished with good efficiency. Since thedivided flows of the resin are gathered later, it is possible tomaintain the entire resin at the foaming temperature, and foams of goodquality can be obtained. Furthermore, since larger amounts of the resinextruded from the extruder can be cooled and maintained at the desiredfoaming temperature, foams of the resin can be produced with goodproductive efficiency.

Examples of the thermopoastic resin to which the apparatus of thepresent invention can be applied include, for instance, polyethylene,polypropylene, polystyrene, polyvinyl chloride, ethylene/vinyl acetatecopolymer, and ethylene/vinylchloride copolymer. The blowing agents arepetroleum ether, propane, butane, pentane, hexane,monochlorodifluoromethane, dichlorodifluoromethane,dichlorotetrafiuoroethane, monochloropentafluoroethane, etc and inertgas such as nitrogen gas, carbon dioxide.

The invention will be described in further detail with reference to theaccompanying drawings in which:

FIG. 1 is a vertical sectional view showing one example of the extruderused in the present invention;

FIG. 2 is a vertical sectional view showing another example of theextruder used in the present invention;

FIG. 3 is a vertical sectional view showing still another example of theextruder used in the present invention;

FIG. 4 is a vertical sectional view showing still another example of theextruder used in the present invention;

FIG. 5 is an enlarged elevational view showing the dulmage type mixingtorpedo fitted in the extruder shown in FIG. 4;

FIG. 6 is a transverse sectional view taken along the line VI-Vl of FIG.5;

FIG. 7 is an enlarged transverse sectional view showing an injectionport for a blowing agent provided in the extruder illustrated in FIG. 4;

FIG. 8 is a transverse sectional view showing one example of the priorart extruder;

FIG. 9 is a vertical sectional view showing one example of the coolingdevice of the apparatus of the invention;

FIG. 10 is a transverse sectional view taken along the line X-X of FIG.9;

FIG. 11 is a vertical sectional view showing another example of thecooling device of the apparatus of the invention;

FIG. I2 is a transverse sectional view taken along the line XII-XII ofFIG. 11; and

FIG. 13 is a transverse sectional view taken along the line XIII-XIII ofFIG. 11.

Referring to FIG. 1, an 1 screw 1 is fitted in an extruder barrel 2. Theextruder screw 1 has a screw flight 3, a screw root 4, and a screwchannel 5, and consists of a compression zone I and and a metering zoneII folpitch in the compression zone I and the metering zone II isconstant, respectively. The depth of screw channel in the compressionzone I decreases progressively in the extruding direction, and the depthof screw channel in the metering zone II is 2.0 times that of the finalscrew channel of the compression zone I. Thus, a volume in unit pitch ofthe screw in the compression zone I decreases progressively in theextrusion direction. A volume of unit pitch of the screw in the meteringzone Ii is constant within the range of 1.6 2.0 times a volume in thefinal unit pitch of the screw in the compression zone I and constant inthe extrusion direction.

The injection port 6 is provided at that part of the extruder barrelwhich corresponds to the first /2-l pitch of the metering zone II of theextruder screw 1. A blowing agent is injected through an injection tube7. The clearance between the extruder diameter and the extruder barrelbore is generally in the range of 0.075 to 0.1 mm. A feed hopper isdesignated at 8, and the material is fed into the extruder through afeed opening 9. The extruder barrel can be heated by means of barrelheaters 10. The reference numeral 15 represents an inlet for coolingwater to be supplied inside the extruder screw 1, and the numeral 16represents its outlet. Towards the end of the extruder is located abreaker plate 11, and an adapter is secured to the flange of theextruder by means of a bolt 13 so as to connect an extrusion die to theforward end of the extruder.

In FIG. 2, an intermediate metering zone III is disposed between thecompression zone I and the metering zone II of the screw 1. The screwpitch of the inter mediate metering zone III is the same as the screwpitch of the compression zone I and the metering zone II, and isconstant. The depth of screw channel in the intermediate metering zoneIII is equal to the depth of the last screw channel of the compressionzone I. Consequently, a volume in unit pitch of the screw in theintermediate metering zone III is ewual to a volume in the final unitpitch of the screw of the compression zone I.

Referring to FIG. 3, the first portion of the metering zone II of theextruder of the type illustrated in FIG. 1 is formed of a dulmage typemixing torpedo 17. In FIG. 4, the first portion of the metering zone ofthe extruder of the type disclosed in FIG. 2 is formed of a dulmage typemixing torpedo 17. As illustrated in FIGS. 5 and 6, the dulmage typemixing torpedo 17 has circumferential necks l8, rounded grooves 19,lands 20 between the grooves 19, and studs 21 for connecting the screw.The reference numeral 22 is a hole through which cooling water passes. Avolume between one circumferential neck 18 and the subsequentcircumferential neck 18 of the dulmage type mixing torpedo l7, i.e., avolume in unit pitch, is equal to a volume in unit pitch of the screw inthe metering zone II.

The injection port 6 for supplying a blowing agent is provided at thatpart of the extruder barrel 1 which corresponds to the first /2-l pitchof the dulmage type mixing torpedo. The injection tube 7 includes ballcheck valves 23 and 24, as shown in FIG. 7, for preventing the back flowof blowing agent injected.

In FIGS. 9 and 10, a cooling device is designated at 25. A coolantvessel 26 is disposed coaxially with the extruder and has side wallplates 27 and 28. Heaters 29 and an impeller 30 are provided within thecoolant vessel 26 so as to maintain a coolant, such as water, oil andglycerine, filled in the coolant vessel 26 at a constant temperature. Aplurality of pipes 31 for dividing the flow of molten resin extend inthe extruding direction through the coolant vessel 26 from one wallplate 27 to the other wall plate 28, and are spaced at equal intervals.An adapter 33 is secured to the wall plate 27 by means of a bolt 32 fordividing the flow of molten resin extruded from the extruder and causethe divided flows to pass through the pipes 31. To the other plate 28 ofthe coolant vessel 26 is fixed an adapter 35 by means of a bolt 34 forgathering the flows of resin extruded from the pipes 31. The end of theadapter 33 is secured to the end of the extruder by means of a bolt 36.To the end of the adapter 35 is fixed an adapter 12 for connecting anextrusion die, by means of a bolt 37, and an extrusion die (not shown)is connected to the adapter 12.

FIGS. 11 to 13 show other examples of the cooling device. The referencenumeral 38 represents a cylinder, both ends of which are closed withwall plates 27 and 28, respectively. A plurality of pipes 31 fordividing the flow of resin extend through the cylinder 38 in theextruding direction from the wall plate 27 to the other wall plate 28,and are spaced at equal intervals. A pipe 39 for jetting out a coolanthaving a jetting port 40 is disposed within the cylinder 38 along thecentral axis of the cylinder 38 and extends from the wall plate 27 tothe other wall plate 28. An inlet for a coolant which extends from oneend of the pipe 39 and through the wall plate 27 and opens into theoutside is designated at 41, and the reference numeral 42 represents anoutlet for the coolant which opens from the inside of the cylinder 38into the outside thereof. A torpedo 43 is provided to smoothen the flowof resin. A coolant, such as water, oil, and glycerine, maintained at apredetermined temperature is fed to the coolant jetting pipe 39 from theinlet 41 and through the wall plates of the cylinder 38, and jetted outfrom the jetting apertures 40 into the inside of the cylinder 38 therebyto cool the pipes 31 at a predetermined temperature. The coolant is thencirculated to the outlet 42.

Illustrative examples of the present invention will be given below.

EXAMPLE 1 The apparatus used in this Example had the followingconstruction.

An extruder screw having a screw diameter of 65 mm. a constant screwpitch of 65 mm, an exial flight land width of 8 mm, and a helix angle of1740 is fitted in an extruder barrel having an inner diameter of about65 mm. The extrusion screw consists of a tapered compression zone Ihaving the depth of the first screw channel of 9 mm and the depth of thelast screw channel of 3 mm and a length of 650 mm, an intermediatemetering zone 111 following said compression zone I, which has a screwchannel depth of 3 mm constantly along its length and a length of 260mm, and a metering zone 11 following the intermediate metering zone,which has a screw channel depth of 6 mm and a length of 1,170 mm. Aninjection port for injecting a blowing agent is provided on that portionof the extruder barrel which corresponds to the first fa pitch of themetering zone. An extrusion die having a nozzle with a diameter of 16 mmis fitted to the forward end of the extruder having the above-mentionedconstruction. This apparatus is of the type illustrated in FIG. 2 of theaccompanying drawings.

Using the extrusion apparatus of the above construction, an intimatemixture of parts by weight of low density polyethylene pellets having amelt index of 2.0 and 0.5 part by weight of talc was fed to the hopperof the extruder, and melted by heating at 140C. in the compression zoneI and the intermediate metering zone III. The speed of the extruderscrew was adjusted to 20 rpm., and the molten mixture was transferred tothe metering zone 11 by the rotation of the extrusion screw.Dichlorodifluoromethane was continuously injected from an injection portof the metering zone II at a rate of 20 percent by weight based on themolten mixture. The extruder screw and the extruder barrel were cooledwith cooling water, and the temperature of the mixture in the meteringzone 11 after the injection port was adjusted to near 1 10C. which waswithin an optimum foaming temperature range. The mixture was extrudedfrom the nozzle of the extrusion die to form a good quality foam havinga circular section.

The injecting pressure of the blowing agent, output rate of the mixture,the extrusion temperature of the mixture, the density of the obtainedfoam, and the properties of the foam were measured, and the resultsobtained are shown in Table l.

COMPARATIVE EXAMPLE 1 The procedure set forth in Example 1 was repeatedexcept that there was used an apparatus of the type illustrated in FIG.8 of the accompanying drawings which includes an extruder screw having ascrew diameter of 65 mm, a screw pitch of 65 mm, an axial flight landwidth of8 mm and a helix angle of 1740, said extruder screw consistingof a tapered compression zone A having the first screw channel depth of9 mm and the last screw channel depth of 3 mm and a length of 650 mm, anintermediate metering zone B following said compression zone I andhaving a constant screw channel depth of 3 mm and a length of 260 mm, anenlarged zone C consisting of a parallel portion having a constant screwchannel depth of 10 mm and a length of 130 mm and a tapered portionfollowing said parallel portion and having the first screw channel depthof 10 mm and the final screw channel depth of 4.5 mm and a length of 260mm, and a metering zone D following said enlarged zone C and having aconstant screw channel depth of 4.5 mm and a length of 780 mm.

The same measurements as in Example I were conducted with respect to theresulting foam product, and the results are shown in Table 1.

EXAMPLE 2 When the screw speed in Example I was made more than 40 rpm toincrease the output of the resin mixture, the temperature of the mixturerose in spite of cooling of the extruder screw and barrel, and it wasdifficult to maintain the mixture at a temperature suitable for foaming.Accordingly, a cooling device of the type illustrated in FIG. 11 wasfitted between the extruder and the extrusion die. The screw speed wasadjusted to 60 rpm, and glycerine heated at C. was circulated from theinlet to the outlet of the cooling device so as to cool a plurality ofpipes having an inner diameter of 18 mm and a length of 750 mm fordividing the flow of the resin mixture. Thus, a foamed product of goodquality was obtained.

The same measurements as set forth in Example 1 were conducted, and theresults are shown in Table 1.

TABLE 1 Comparative Example l Example 2 Example Screw speed (r.p.m.) 2060 Injection pressure of SOfl 55:2 70:2 a blowing agent (kg/cm) Outputrate of the 1531.5 1510.3 40:1.2 mixture (kg/hr.)

Extrusion H5 (at the 1 l5 (at the 145 (at the temperature of the outletof the outlet of the outlet of the mixture (C.) extruder) extruder)extruder) l 14 (at the outlet of the cooling device) Density of the foam006010.005 004010.002 003510.002 (g/cm") Properties of the Article sizeArticle size Article size resulting foam nonuniform uniform with uniformwith with uniform fine uniform fine nonuniform cells cells coarse cellsWhat is claimed is:

1. An extrusion apparatus for use in the production of a thermoplasticresin foam including an extruder comprising an extruder barrel, acontinuous extruder screw fitted in said extruder barrel and aninjection port for blowing agent provided in said extruder barrel, saidextruder screw consisting of a compression zone wherein the volume inunit pitch of said screw decreases progressively in the extrusiondirection and a metering zone wherein the volume in unit pitch of saidscrew is 1.6 2.0 times the volume in the final unit pitch of said screwin the compression zone and is constant in the extrusion direction, saidinjection port being provided at that portion of said extruder barrelwhich corresponds to the metering zone.

2. The extrusion apparatus of claim 1 wherein an intermediate meteringzone in which the volume in unit pitch of the screw is equal to thevolume in the final unit pitch of the screw of the compression zone isdisposed between the compression zone and the metering zone.

3. The extrusion apparatus of claim 1 wherein the unit pitch of thescrew in the compression zone and the metering zone is constant.

4. The extrusion apparatus of claim 2 wherein the unit screw pitch ofthe compression zone, intermediate metering zone and metering zone isconstant.

5. The extrusion apparatus of claim 1 wherein said injection port forblowing agent is provided in that portion of the extruder barrel whichcorresponds to the first /2-1 pitch of the metering zone.

6. The extrusion apparatus of claim 2 wherein said injection port forblowing agent is provided in that part of the extruder barrel whichcorresponds to the first -l pitch of the metering zone.

v UNITED STATES PATENT QFFICE E T E OF CORRECTION Patent No. 3 82 7 841Dated August 6, 1974 Inventofl s) IsamiRAWAI ET AL It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

' In the Heading, insert patentees' foreign priority data as follows:

-- Japanese Application 'No. 50187/69, filed June 24, l969, end

Japanese Application No. 83124/69, filed Qetober 16 1969.

I Signed and sealed this 7thdayof January 1975- f (SEAL) Attesin p v sMcCoy L1.GIBSONJR. f c. MARSHALL DANN Attesting Officer? n. Commissionerof Patents FORM Po-wso (o-s9) v 'USCOMMDC 503 6-P59 ".5. GOVERNHENYPRINTING OFFICE {9C9 O-J66-33l

1. An extrusion apparatus for use in the production of a thermoplasticresin foam including an extruder comprising an extruder barrel, acontinuous extruder screw fitted in said extruder barrel and aninjection port for blowing agent provided in said extruder barrel, saidextruder screw consisting of a compression zone wherein the volume inunit pitch of said screw decreases progressively in the extrusiondirection and a metering zone wherein the volume in unit pitch of saidscrew is 1.6 - 2.0 times the volume in the final unit pitch of saidscrew in the compression zone and is constant in the extrusiondirection, said injection port being provided at that portion of saidextruder barrel which corresponds to the metering zone.
 2. The extrusionapparatus of claim 1 wherein an intermediate metering zone in which thevolume in unit pitch of the screw is equal to the volume in the finalunit pitch of the screw of the compression zone is disposed between thecompression zone and the metering zone.
 3. The extrusion apparatus ofclaim 1 wherein the unit pitch of the screw in the compression zone andthe metering zone is constant.
 4. The extrusion apparatus of claim 2wherein the unit screw pitch of the compression zone, intermediatemetering zone and metering zone is constant.
 5. The extrusion apparatusof claim 1 wherein said injection port for blowing agent is provided inthat portion of the extruder barrel which corresponds to the first 1/2-1 pitch of the metering zone.
 6. The extrusion apparatus of claim 2wherein said injection port for blowing agent is provided in that partof the extruder barrel which corresponds to the first 1/2 -1 pitch ofthe metering zone.